mix alloy soldering (sn/pb soldering in a pb free world) alloy soldering (sn/pb soldering in a pb...

Mix Alloy Soldering

(Sn/Pb Soldering in a Pb Free World)Dr. Neil Poole

January 2007

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Why Backward Why Backward Compatibility?Compatibility?l Some electronics products have exemptions in EU RoHS

directiveLead in solders in high reliability applications such as servers, medicalequipment, telecommunication products until 2010 or beyond.USA has no Pb free legislation (some states looking CA)

l Some SnPb components will be unavailable or command a high premium

Component manufacturers do not want to carry both SnPb andlead-free production lines due to the cost concern

Volume of components for these high reliability products is low

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IntermetallicIntermetallic Bonding (Soldering).Bonding (Soldering).

SUBSTRATE INTERMETALLIC (compound)

Cu Cu6 Sn

Ag Ag3 Sn

Au Au Sn4 (WEAK, BRITTLE)

Ni Ni3 Sn4

- TIN IS THE ‘ACTIVE INGREDIENT’ IN SOLDER -

SOLDERSOLDER

INTERMETALLICINTERMETALLIC LAYERLAYER

SUBSTRATESUBSTRATE

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The The IntermetallicIntermetallic LayerLayerThickness of intermetallic layer depends on: -

l Substrate compositionAu > Ag > Cu > Pd > Ni

l Solder compositionl Temperature!l Time!

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““Mixed AlloyMixed Alloy”” SystemsSystems

l Pb free lead finishes with Pb solderLittle work done so farPossible profile changes

l Pb lead finishes with Pb free solderSome work doneSeen as a transitional problem

l Even with exemptions full Sn/Pb may not be possible

l Mistakes will happen

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““Mixed AlloyMixed Alloy”” SystemsSystemsPbPb--free Component Finishesfree Component Finishes

l Pure SnMost commonWhiskers(?)

l Sn/BiMore popular in JapanReliability Questions

l Ni/Pd/AuVendor SpecificQuestions about wettability

l Sn/Pb + Pure SnWetting issuesReflow Profile

l Sn/Pb + Sn/BiSn/Bi/Pb – low melt temp

l Sn/Pb + Ni/Pd/AuLeast affected

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Alloy Melting PointsAlloy Melting Points

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IntermetallicIntermetallic Thickness Thickness

Cu3SnCu6Sn3Cu3SnCu6Sn3

Sn62 AlloySAC 305 AlloyTreatmentFinish

0.51.01.03.0As Reflowed

Cu

1.53125ºC for 1000 hr

ENIC

12.5As Reflowed

ENIC

0.53.51.04.0125ºC for 1000 hr

Im Ag

0.51.00.03.0As Reflowed

Im Ag

0.53.51.04.0125ºC for 1000 hr

Cu

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Wetting Speed Wetting Speed vsvs Temperature for Temperature for Different AlloysDifferent Alloys

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

200 210 220 230 240 250 260 270 280

Test temperature, °C

tim

e to

2/3

max

wet

tin

g f

orc

e, s

60/40

SnCu base alloys

SnAg & SnAgCu base alloys

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Comparative WettingComparative WettingComparative Wetting Times for Pb Free Terminations

For Sn62 and SAC 387 Alloys

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Comparative WettingComparative Wetting

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N/AN/AIm Sn

N/AN/AIm Ag

10.3 Kg9.7 KgENIC

10.0Kg9.5 KgCopper

Peak Reflow Temperature 240ºC

Peak Reflow Temperature 215ºCPad Finish


Shear testing

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Au 240ºC

Au 215ºC

Cu 215ºC


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0402

215ºC 240ºC

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““Mixed AlloyMixed Alloy”” SystemsSystemsPbPb--free free ““LeadedLeaded”” Component FinishesComponent Finishes

215ºC 240ºC

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““Mixed AlloyMixed Alloy”” SystemsSystemsPbPb--free free ““LeadedLeaded”” Component FinishesComponent Finishes

Au 215ºC Au 240ºC

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Conclusions Leaded and Conclusions Leaded and Leadless ComponentsLeadless Components

l Process temperature can influence joint microstructureHigher peak temperature can increase Ag3Sn dendritesHigher peak temperature can increase Pb phaseHigher peak temperature increase initial intermatallic thicknessHigher temperature reduces voidingHigher peak temperatures better wetting (leaded components)

l Initial joint strengths independent of the profile l Effects on reliability still to be determined!!!!

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““Mixed AlloyMixed Alloy”” SystemsSystemsPbPb--free Bumped Component Finishesfree Bumped Component Finishes

l SAC AlloysSAC 305SAC 405SAC 387SAC 105??

l SACs 305, 387, 405 appear to be equalIPC Solder Value Council

l Many, Many, Many QuestionsUnderfill!!!

All essentially eutectic melt point 217ºC

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l Pb-free BGA bumps / Sn/Pb Pastel Metals in the solder joint:

Sn (bulk)AgCuPbNiAu

l Not evenly distributed not necessarily eutectic structure


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Bumped Component ChallengeBumped Component Challenge

• What reflow profile??

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l Outer Appearance – BGA bumps

New BGA


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l Outer Appearance – BGA bumps

Lead paste Reflowed@ 210 C

Ball did not melt


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l Pb-free BGA bumps / Sn/Pb Paste & ProfileBump doesn’t melt/collapse

Component at a higher standoffStress distribution differentComponent tiltImpact to reliability

Segregated joint


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l Pb-free BGA bumps & Profile / Sn/Pb PasteFlux exhaustion of the Sn/Pb Paste

Poor coalesanceIncreased voiding/blowholes

Insufficient activityPoorer wetting

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Mixed Alloy Reflowed BGAMixed Alloy Reflowed BGA

Rockwell

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Mixed Alloy Reflowed BGAMixed Alloy Reflowed BGA

Thermal Cycling-55 / +125

10C/min, 11 min dwell“Normal” Sn/Pb= 1000+

Mixed Metal = 267

Rockwell

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““Mixed AlloyMixed Alloy”” Reflowed BGAReflowed BGA

Ag3Sn Platelet inside the void indicates that the void formed during the soldering process or during cooling.

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Bump collapsed and part self centeredFull mixing of alloys.

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Micro Structure of Joints with Micro Structure of Joints with Low Levels of PbLow Levels of Pb

0.2% Pb

4.0% Pb

Forsten, Steen, & Wilding, Soldering & Surface Mount Technology

Pb-rich phase, 179°C melting temperature

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Final Joint Compositions & Final Joint Compositions & LiquidusLiquidus for for Sn3Ag0.5CuSn3Ag0.5Cu Ball and Sn37Pb PasteBall and Sn37Pb Paste

2080.32.111.91090411 (Sq)0.5

203(200*)

0.31.617.01080514 (Sq)0.65

2090.42.211.11485516 (Sq)0.8

2150.42.64.92290518 (Sq)1.0

2160.52.82.9281005

2160.52.73.428100621 (Ro)1.27

Liquidus temp. (C)

% of Cu

% of Ag

% of Pb Ball Dia. (mil)

Transfer Ratio (%)

Stencil Thickness (mil)

Aperture Size (mil)

Pitch (mm)

J. Pan, et al., IPC/JEDEC Pb-free Conference, San Jose, 2006.* Calculated by Dr. Ursula R. Kattner of NIST from NIST thermodynamic database

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Final Joint Compositions & Final Joint Compositions & LiquidusLiquidus for for Sn4Ag0.5Cu Ball and Sn37Pb PasteSn4Ag0.5Cu Ball and Sn37Pb Paste

2060.32.711.91090411 (Sq)0.5

201(197*)

0.32.217.01080514 (Sq)0.65

2070.42.911.11485516 (Sq)0.8

2170.43.54.92290518 (Sq)1.0

218*0.53.72.9281005

2180.53.63.428100621 (Ro)1.27

Liquidustemp. (C)

% of Cu% of Ag% of Pb Ball Dia. (mil)

Transfer Ratio (%)

Stencil Thickness

(mil)

Aperture Size (mil)

Pitch (mm)

J. Pan, et al., IPC/JEDEC Pb-free Conference, San Jose, 2006.* Calculated by Dr. Ursula R. Kattner of NIST from NIST thermodynamic database

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““Mixed AlloyMixed Alloy”” ReliabilityReliability-- Thermal CycleThermal Cycle

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Effect of Effect of UnderfillUnderfill on BGS/on BGS/CSPCSP’’ss

1.00 1000.0010.00 100.001.00

5.00

10.00

50.00

90.00

100.00 ReliaSoft's Weibull++ 6.0 Drop Testing .5mm CSP

Drop Count

% Failure

12/19/2006 16:28HENKEL

Tom White

Underfill B 0.5 SAC Drop

Underfill B0.5 SnPb Drop

Underfill A 0.5 SAC Drop

Underfill A 0.5 SnPb Drop

(2900 Gs, 0.3 msec Pulse)

?

?

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ConclusionsConclusionsl There are successes with mixed metal systemsl Reliability window is reduced (underfill?)

Much work still to be donel iNEMI committee looking into mixed metal BGA/CSP

assemblyl Other companies also investigatingl Avoid it if possible, understand the risks if required

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SourcesSources

l Y. Kariya et al: J. Electronic Materials, 1998, 27, p1229-1235l M. Harrison et al: Soldering & Surface Mount Technology, vol 13 no 3, p21-

38l NCMS Report no 0096RE01, 2001l M. Bozack: Auburn University, 2002l G. Swan et al: Proceedings APEX, 2001, paper LF2-6l A. Syed: Proceedings APEX, 2001, paper LF2-7l D. R. Banks et al: Proc SMI conf, 1996, p121-126

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AcknowledgementsAcknowledgementsl Dr. Neil Poole – Sr. Applications Chemistl Dr. Hector Steen – Sr. Development Scientistl David Hillman – Rockwell Collinsl Paul Wood – OK Industriesl Ramon Mendez – Elcoteql Tom White – Henkell Pauline Chan – Henkell Dr. Jainbiao (John) Pan - Assistant Professor Cal Poly State University,

San Luis Obispo.

Any Questions

Dr. Neil Poole

mix alloy soldering (sn/pb soldering in a pb free world) alloy soldering (sn/pb soldering in a pb...

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